Dr. Catherine Clark, Chapman University – Sunlight and Water Pollution

In today’s Academic Minute, Dr. Catherine Clark of Chapman University explains the connection between sunlight and water pollution.

Catherine Clark is a professor of earth and environmental sciences at Chapman University in Orange, California. Her research is primarily focused on aquatic chemistry, specifically the photochemistry of dissolved organic material in coastal and ocean waters and the sources, distribution and photochemical reactions of pollutants in local waters. She holds a Ph. D. in chemistry from Boston University.

Hydrogen peroxide is a reactive molecule with two hydrogen and two oxygen atoms. You are probably familiar with peroxide in hair bleach and whitening toothpaste, but it is also produced in water when sunlight interacts with natural organic matter or NOM. NOM is a complex, large molecule produced from decaying plant material. It absorbs sunlight to become an energized state which reacts with dissolved oxygen to produce peroxide after a series of reactions.

We are studying this process in seawater at popular swimming and surfing beaches in Southern California. Concentrations are typically about a million times lower than the commercial product you can buy at the drugstore, but even at such low concentrations, the peroxide still has significant impacts on the environment by degrading pollutants. It can do this in 3 ways; by reacting with other molecules and transferring electrons in redox processes, the same kind of reactions involved in the rusting of iron; or by producing radical species with unpaired electrons which react rapidly with organic molecules; and lastly, by killing bacteria. In fact, peroxide is often added to disinfect waste water at sewage treatment plants.

Beach pollution comes from urban runoff and can cause sickness and beach closures. Runoff is produced when water from rain or sprinklers runs over surfaces like roads and lawns and washes pollutants into the ocean via storm drains. Runoff carries many kinds of pollution, from fecal bacteria like E. Coli from people who don’t pick up their dog waste to PAHs which are cancer- causing organic compounds formed when fossil fuels burn.

Peroxide levels in beach water increase during the day as the sun rises, and decay at night. The opposite is observed for pollutants like fecal coliform bacteria, which are highest in the early morning and lowest after noon. From a chemical point of view, it is best to swim between12p.m. and 2 p.m. when the sun is strongest Because the rising peroxide levels protect against fecal indicator bacteria. However, this is the worst time for sunburn so wear lots of sunscreen!